U.S. Pat. No. 4,705,190 discloses a closure for a tank wherein a cap is held on a holding unit made of plastic so that the cap cannot be lost when threadably disengaged from the tank. The holding unit comprises a ring-like portion at a first end which surrounds the cap. For this reason, the ring-like portion must be made correspondingly large and the second end of the holding unit is insertably held in a tube. A retainer in the form of a projection is provided at the second end of the holding unit and prevents the holding unit from slipping out of the tube. This holding unit for a tank closure is configured for motor vehicles and is complex to manufacture. In addition, the holding unit requires additional construction space for accommodating the insertable portion of the holding unit. A loop is formed laterally of the tank closure and would be most inconvenient in portable handheld tools when working, for example, in brush or the like.